Systems for remote identification of objects are useful for many purposes, including identifying and locating objects such as trains or automobiles. Such systems use RF signals to communicate information between a reader apparatus and a transponder attached to the object. Each transponder has an individual code containing information related to and identifying the object to which it is attached. The reader sends an RF signal to the remote transponder. An antenna in the transponder receives the signal from the reader, backscatter-modulates the received signal with data temporarily or permanently stored in the transponder, thereby producing a sequence of signals in accordance with the transponder's individual code, and reflects this modulated signal back to the reader to pass the information contained in the transponder to the reader. The reader decodes these signals to obtain the information from the transponder. Likewise, the transponder may decode signals received from the reader and write information to the transponder's memory. The details of these transponders and readers have been previously described in U.S. Pat. No. 4,739,328, which is hereby incorporated by reference.
However, the system described in U.S. Pat. No. 4,739,328 provides no information about the speed and movement of the transponder with respect to the reader. While there are known techniques for determining relative movement, those techniques are unsatisfactory for use in the reader/transponder system described above. For example, a common Doppler radar can only sense velocity and direction of movement above some minimum relative velocity and it cannot sense when a target object is stationary. Also, Doppler radars typically use a low frequency range that is incompatible with the reader/transponder systems in which this invention is intended to function. Koelle and Depp described another technique in an article entitled, "Doppler Radar with Cooperative Target Measures to Zero Velocity and Senses the Direction of Motion," in Proceedings of the IEEE, March 1977. However, that technique requires use of additive and subtractive phase terms and is also incompatible with present reader/transponder systems that use frequency shift keying or biphase codes to convey information between the reader and transponder. Other techniques have been described using a receiver with more than one antenna, as in U.S. Pat. No. 4,728,955.